Method for realizing microchanels in an integrated structure

ABSTRACT

A process is presented for realizing buried microchannels ( 10 ) in an integrated structure ( 1 ) comprising a monocrystalline silicon substrate ( 2 ). The process forms in the substrate ( 2 ) at least one trench ( 4 ). A microchannel ( 10 ) is obtained starting from a small surface port of the trench ( 4 ) by anisotropic etching of the trench. The microchannel ( 10 ) is then completely buried in the substrate ( 2 ) by growing a microcrystalline structure to enclose the small surface port.

PRIORITY CLAIM

[0001] The present application claims priority from European Applicationfor Patent No. 02425746.1 filed Dec. 4, 2002, the disclosure of which ishereby incorporated by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Technical Field of the Invention

[0003] The present invention relates to a process for realizingmicrochannels in an integrated structure. More specifically, theinvention relates to a process for realizing microchannels buried in anintegrated structure comprising a monocrystalline silicon substrate.

[0004] 2. Description of Related Art

[0005] Typical procedures for analyzing biological materials, such asnucleic acid, involve a variety of operations starting from rawmaterial. These operations may include various degrees of cellpurification, lysis, amplification or purification, and analysis of theresulting amplification or purification product.

[0006] As an example, in DNA-based blood tests the samples are oftenpurified by filtration, centrifugation or by electrophoresis so as toeliminate all the non-nucleated cells. Then, the remaining white bloodcells are lysed using chemical, thermal or biochemical means in order toliberate the DNA to be analyzed. Next, the DNA is denatured by thermal,biochemical or chemical processes and amplified by an amplificationreaction, such as PCR (polymerase chain reaction), LCR (ligase chainreaction), SDA (strand displacement amplification), TMA(transcription-mediated amplification), RCA (rolling circleamplification), and the like. The amplification step allows the operatorto avoid purification of the DNA being studied because the amplifiedproduct greatly exceeds the starting DNA in the sample.

[0007] The procedures are similar if RNA is to be analyzed, but moreemphasis is placed on purification or other means to protect the labileRNA molecule. RNA is usually copied into DNA (cDNA) and then theanalysis proceeds as described for DNA.

[0008] Finally, the amplification product undergoes some type ofanalysis, usually based on sequence or size or some combination thereof.In an analysis by hybridization, for example, the amplified DNA ispassed over a plurality of detectors made up of individualoligonucleotide detector “probes” that are anchored, for example, onelectrodes. If the amplified DNA strands are complementary to theprobes, stable bonds will be formed between them and the hybridizeddetectors can be read by observation by a wide variety of means,including optical, electrical, magnetic, mechanical or thermal means.

[0009] Other biological molecules are analyzed in a similar way, buttypically molecule purification is substituted for amplification anddetection methods vary according to the molecule being detected. Forexample, a common diagnostic involves the detection of a specificprotein by binding to its antibody or by a specific enzymatic reaction.Lipids, carbohydrates, drugs and small molecules from biological fluidsare processed in similar ways. However, we have simplified thediscussion herein by focusing on nucleic acid analysis, in particularDNA amplification, as an example of a biological molecule that can beanalyzed using the devices of the invention.

[0010] The steps of nucleic acid analysis described above are currentlyperformed using different devices, each of which presides over oneaspect of the process. The use of separate devices increases cost anddecreases the efficiency of sample processing because transfer timebetween devices is required, larger samples are required to accommodatesample loss and instrument size, and because qualified operators arerequired to avoid contamination problems. For these reasons anintegrated microreactor would be preferred.

[0011] For performing treatment of fluids, integrated microreactors ofsemiconductor material are already known. Microchannel arrays are widelyused in different systems such as medical systems for fluidadministration, devices for biological use for manufacturingminiaturized microreactors, in electrophoresis processes, in DNA chipand other array applications, in integrated fuel cells, ink jetprinters, and the like. Microchannels are used also, for example, forthe refrigeration of devices located above microchannels.

[0012] One application of interest is the use of microchannels to make aminiaturized microreactor for diagnostic uses (see especially, U.S. Ser.No. 10/663,268 filed Sep. 16, 2003 and references cited therein, eachwhich incorporated by reference in their entirety). A number of suchdevices are described for the amplification of nucleic acid, such as DNAor RNA, or for other biological tests, such as immunological detectionof antigens in a biological sample. The microreactor can be combinedwith one or more integrated sample pretreatment chamber, micropump,heater, and also with integrated sample analysis features, such as anarray of nucleic acid or antibody detectors. Such devices are describedin more detail in U.S. Ser. No. 10/663,268, and related patents orapplications.

[0013] However, complex procedures are traditionally required in orderto form a microchannel system. In particular, conventional processes forforming embedded microchannels require so-called wafer bonding oropening structures from the backside of the wafer back.

[0014] A process for forming microchannels is described for example inthe U.S. Pat. No. 6,376,291 granted on Apr. 23, 2002. In particular,this document describes a process for forming in a monocrystallinesilicon body an etching-aid region for the monocrystalline siliconwherein a nucleus region is provided, surrounded by a protectivestructure and having a port extending along the whole etching-aidregion.

[0015] According to the '291 patent, a polycrystalline layer is grownabove the port in order to form a cavity completely embedded in theresulting wafer. Although advantageous from many aspects, the processdescribed by the '291 patent is rather complex and it does not allow acompletely crystalline final microstructure to be obtained.

[0016] The technical problem underlying the present invention is toprovide a process for forming microchannels, having such structural andfunctional characteristics as to overcome the limits and drawbacks stillaffecting the processes according to the prior art.

SUMMARY OF THE INVENTION

[0017] The solution underlying the present invention is to use trenchstructures to obtain deep silicon cavities. A small surface port is usedas a precursor for forming microchannels in an integrated structure.Through the port, a trench is defined and then etched to form themicrochannel structure. The port is then closed by silicon to thusobtain a completely crystalline final structure.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] A more complete understanding of the method and apparatus of thepresent invention may be acquired by reference to the following DetailedDescription when taken in conjunction with the accompanying Drawingswherein:

[0019]FIG. 1 schematically shows a section of an integrated structurewith at least a microchannel realized with the process according to theinvention;

[0020]FIGS. 2, 3A, 3B and 4 are micrographs of the integrated structureof FIG. 1 in different steps of the process according to the invention;

[0021]FIG. 5 schematically shows an integrated structure withmicrochannels realized according to an alternative embodiment of theprocess according to the invention;

[0022]FIGS. 6A-6F schematically show an integrated structure withmicrochannels in different steps of a further alternative embodiment ofthe process according to the invention; and

[0023]FIGS. 7A and 7B show micrographs of the final integrated structurewith microchannels realized with the process according to the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

[0024] The invention relates particularly, but not exclusively, to aprocess for realizing miniaturized microchannels buried in a completelymonocrystalline array and the following description is made withreference to this field of application for convenience of illustrationonly.

[0025] With reference to the drawings, and particularly to FIG. 1, anintegrated structure comprising a plurality of microchannels 10 formedaccording to the invention is globally and schematically indicated withreference 1.

[0026] In particular, the integrated structure 1 comprises amonocrystalline silicon substrate 2 whereon a monocrystalline siliconlayer 3 is grown.

[0027] The monocrystalline silicon layer 3 is obtained in turn byepitaxial growth on convenient cavities (rhombohedral in the exampleshown) of said microchannels 10 without using coverings.

[0028] Advantageously, according to the invention, microchannels 10 arecompletely buried in the substrate 2 and the final integrated structure1 is completely monocrystalline.

[0029] The steps of the process according to the invention for formingburied microchannels 10 in a completely monocrystalline integratedstructure 1 are now described. As it will be seen in the followingdescription, advantageously, according to the invention, theseminiaturized channels are completely obtained through surfacemicromachining processes.

[0030] The process for forming buried microchannels 10 in an integratedstructure 1 according to the invention comprises the steps of:

[0031] providing a monocrystalline silicon substrate 2;

[0032] forming on the substrate 2 surface a silicon nitride mask (Hardmask) through a CVD deposition technique; and

[0033] opening of a window having a convenient width L throughphotolithographic systems and following plasma etching.

[0034] In particular, as it is schematically shown in FIG. 2, above thesubstrate 2 a window is opened having a width L of about 1 mm and adepth H of about 9 mm along the substrate 2 direction, indicated infigure with the arrow F.

[0035] Advantageously according to the invention, the process provides afollowing plasma etching which uses the hard mask to form deep trenches4 in the substrate 2, as shown in FIG. 2. Trenches 4 have side walls 4Aand 4B which are substantially orthogonal to the substrate 2 surface.

[0036] The resulting structure then undergoes a further anisotropic wetetching, for example with a TMAH or KOH solution.

[0037] It is worth noting that solutions with different KOH or TMAHconcentrations etch the monocrystalline silicon of the substrate 2 withspeeds which highly depend on the crystallographic orientations and thedopant concentration of the substrate 2 itself. It is thus possible, byusing a TMAH- or KOH-solution-etching, to form highly controllable andreproducible three-dimensional microchannels 10.

[0038] Advantageously according to the invention, trenches 4 are theprecursors of microchannels 10.

[0039] The integrated structure 1, after the anisotropic etching step,has the shape shown in FIGS. 3A and 3B, wherein a single microchannel ora plurality of microchannels are shown, respectively.

[0040] Advantageously according to the invention, the resultingmicrochannels 10 have a rhombohedral shape.

[0041] In particular, the original shape of trenches 4 (shown in FIG. 2)turns into a pair of so-called rotated v-grooves V1 and V2, orthogonalto the surface S of the substrate 2 and defining rombohedron-shapedmicrochannels 10, as shown in FIG. 3A.

[0042] In other words, a bottleneck-shaped deep cavity is obtained,which has a small port on the surface S of the substrate 2.

[0043] In practice, while the etching time passes, because of thepresence of a so-called under cut under the hard mask on the substrate 2surface, microchannels 10 open upwardly changing the symmetry betweenthe upper and lower part of their cavity, as schematically shown in FIG.4.

[0044] It is, however, possible, by limiting the etching time, to obtainconveniently-sized microchannels by enlarging the depth of originaltrenches 4. In the alternative, it is possible to exploit the so-calledetch stop effect by using as hard mask a heavily doped monocrystallinelayer, as schematically shown in FIG. 5, wherein the substrate 2 andmicrochannels 10 are covered by a heavily doped hard mask layer capableof reducing under cut effects even when the substrate 2 etching timepasses.

[0045] In a preferred embodiment, the layer 5 has a dopant concentration(for example, boron) higher than 10¹⁹ atoms/cm³.

[0046] It is also possible to use a predeposition on trench 4 walls of alayer of material 6 having a low etching speed (as, for example, thenitride).

[0047] In particular, this alternative embodiment of the processaccording to the invention provides a deposition of a nitride layer 6followed by a plasma etching effective to open a region 7 at the trench4 base, as shown in FIGS. 6A to 6F.

[0048] The process for realizing buried microchannels 10 in anintegrated structure 1 according to this alternative embodiment of theinvention comprises the steps of:

[0049] providing a monocrystalline silicon substrate 2;

[0050] growing a monocrystalline silicon layer 3 above the substrate 2;and forming a mask by means of a photoelectric film 8 above themonocrystalline silicon layer 3, as schematically shown in FIG. 6A.

[0051] The process provides thus the steps of:

[0052] opening a plurality of windows through photolithographic systemsand following plasma etching (FIG. 6B); and

[0053] forming a plurality of trenches 4 in correspondence with theplurality of windows (FIG. 6C).

[0054] Advantageously this alternative embodiment of the processaccording to the invention, provides therefore a deposition step of anitride layer 6 (FIG. 6D), a removing step of the layer 6, an etchingstep of the silicon substrate in a lower part 9 of trenches 4 (FIG. 6E)and a plasma etching step effective to open a plurality of regions 7 atthe trench 4 base (FIG. 6F).

[0055] In particular, the plasma etching step to open regions 7 at thetrench 4 base is activated only in the area wherein the nitride layer 6has been removed. It is essentially a so-called SCREAM process, whereintrench 4 walls are protected to localize the etching only under thetrench base.

[0056] Even using this alternative embodiment of the process accordingto the invention, deep regions 7 are thus obtained, which have however asmall surface opening in correspondence with the opening areas oftrenches 4.

[0057] Advantageously according to the invention, trenches 4 are usedfor an anisotropic etching effective to obtain rhombohedralmicrochannels. The shape obtained is due to the different etching speedsof the different crystallographic directions.

[0058] The side walls 4A and 4B of trenches 4 undergo the etchinganisotropic action and the erosion continues with different etchingspeeds due to the different atom coordination (in terms of bond quantityof silicon atoms directed towards the substrate).

[0059] In particular, atoms on planes of the (100) type havecoordination two (i.e., two bonds directed towards the substrate),whereas atoms on planes of the (111) type have coordination three (i.e.,three bonds directed towards the bulk); that is that they are morebonded.

[0060] Trenches 4 are directed along the directions (110) on the wafersurface of the (100) type. Planes (111) find on the wafer surface justthe direction (110) and they are rotated with respect to the normal tothe surface by about 54.7°.

[0061] In particular two planes are present, which pass in the upperpart of trenches 4 and two planes passing in the lower part. All atomsalong these directions have coordination three.

[0062] Advantageously according to the invention, the process starts byeroding the atoms having the lowest coordination which are characterizedby a higher speed. After reaching the directions of planes (111) passingthrough/from the upper part and through/from the lower part of trenches4, speed decreases by about a hundred times since it finds only atomswith coordination three, therefore it continues with the etching speedof planes (111) as shown in FIGS. 3A and 3B. In particular, amicrochannel 10 opened towards the substrate 2 surface is obtained.

[0063] Advantageously, according to the invention, deep silicon cavitiesare thus obtained, being characterized by a small surface port wheretoit is possible to apply a silicon deposition step to obtain amonocrystalline structure.

[0064] In other words, microchannels 10 have a bottle-section-shaped orrhomohedral precursor (obtained as above described) which is easilyclosed epitaxially in one embodiment or closed by deposition of a layersuch as an oxide, polysilicon, nitride or other convenient material.

[0065] Advantageously according to the invention, the process provides afurther epitaxial new growth step corresponding to the material used toclose the upper part of the microchannel 10, as shown in FIG. 7A. It isthus possible to obtain completely buried monocrystalline siliconmicrochannels 10.

[0066]FIG. 7B shows for completeness the channel profile before (10A)and after (10B) the epitaxial new growth step. It happens thus that themonocrystalline material deposition occurs consistently also inside themicrochannel 10.

[0067] It is also possible to close the upper part of microchannels byusing other deposition techniques such as oxide or polysilicon ornitride deposition.

[0068] In conclusion, the process for realizing microchannels 10 buriedin an integrated structure 1 according to the invention allows, thanksto the resulting etching form, the structure of the microchannel underthe substrate 2 surface to be enlarged, but to keep, at the same time,the etching port small by means of trenches 4. The surface microchannelclosing is thus performed by growing epitaxially the material.

[0069] Advantageously, according an embodiment of the invention, theintegrated structure 1 is completely epitaxial even above microchannels10 and it is performed by exploiting a deep cavity characterized by asmall surface opening, which can be obtained in several kinds ofprocesses, as well as an easy epitaxial new growth of this cavity.

[0070] Although preferred embodiments of the method and apparatus of thepresent invention have been illustrated in the accompanying Drawings anddescribed in the foregoing Detailed Description, it will be understoodthat the invention is not limited to the embodiments disclosed, but iscapable of numerous rearrangements, modifications and substitutionswithout departing from the spirit of the invention as set forth anddefined by the following claims.

What is claimed is:
 1. A process for realizing microchannels buried inan integrated structure comprising a monocrystalline silicon substrate,comprising: forming in said substrate at least a trench; and obtainingsaid microchannels starting from a deep cavity characterized by a smallsurface port obtained through anisotropic etching of said at least onetrench, said microchannels being nearly entirely buried in saidsubstrate in a completely monocrystalline structure.
 2. The processaccording to claim 1: wherein forming comprises: depositing a mask abovesaid substrate; opening of windows having a convenient width; and plasmaetching which uses said mask to form said trenches having side wallsbeing essentially orthogonal to the surface of said substrate; andwherein obtaining comprises: wet anisotropic etching to form, startingfrom said trenches, said microchannels, said anisotropic etching stepproviding different etching speeds due to different atom coordination.3. The process according to claim 2, wherein plasma etching is performedwith a TMAH or KOH solution.
 4. The process according to claim 2,wherein opening the windows having a convenient width is performedthrough photolitographraphy and subsequent plasma etching.
 5. Theprocess according to claim 2, wherein deposition of a mask above saidsubstrate comprises a silicon nitride deposition through the CVDdeposition.
 6. The process according to claim 2, wherein deposition of amask above said substrate comprises a heavily doped monocrystallinelayer deposition.
 7. The process according to claim 6, wherein theheavily doped monocrystalline layer has a dopant concentration higherthan 10¹⁹ atoms/cm³.
 8. The process according to claim 1, furthercomprising a convenient epitaxial new growing operation effective toclose an upper part of said microchannels and completely bury themicrochannels in monocrystalline silicon.
 9. The process according toclaim 1, further comprising an oxide, polysilicon or nitride depositioneffective to close an upper part of said microchannels and completelybury the microchannels.
 10. The process according to claim 1, whereinthe wet anisotropic etching step turns said side walls of said trenchesinto a pair of rotated v-grooves orthogonal to a surface of saidsubstrate and defining rombohedron-shaped microchannels.
 11. The processaccording to claim 1, further comprising depositing a layer of materialhaving a low etching speed.
 12. The process according to claim 11,further comprising plasma etching effective to open a region at a trenchbase.
 13. The process according to claim 11, further comprising removingof said layer and in an etching of said substrate in a lower part ofsaid trenches before said plasma etching step.
 14. An integratedstructure, comprising: at least a monocrystalline silicon substratewherein at least one microchannel is formed which is nearly entirelyburied inside said substrate.
 15. The integrated structure according toclaim 14, wherein the microchannel has a generally rhombohedralcross-sectional shape.
 16. The integrated structure according to claim14, further comprising an epitaxially grown silicon layer above thesilicon substrate to completely enclose the microchannel inmonocrystalline silicon.
 17. The integrated structure according to claim14, further comprising a layer above the silicon substrate to closecompletely enclose the microchannel.
 18. The integrated structureaccording to claim 17, wherein the layer is an oxide, polysilicon ornitride deposition effective to close an upper part of said microchanneland completely bury the microchannel.
 19. A method for formingmicrochannels, comprising: forming a narrow elongated trench in amonocrystalline silicon substrate; performing an anisotropic wet etch ofthe narrow elongated trench to form a microchannel structure having agenerally rhombohedral cross-sectional shape with a top port substratesurface opening; and closing the top port substrate surface opening ofthe microchannel structure to entirely enclose the microchannelstructure.
 20. The method of claim 19 wherein closing comprisesepitaxially growing monocrystalline silicon on a surface of thesubstrate to entirely enclose the microchannel structure inmonocrystalline silicon.
 21. The method of claim 19 wherein theanisotropic wet etch is made using a TMAH solution.
 22. The method ofclaim 19 wherein the anisotropic wet etch is made using a KHOH solution.23. The method of claim 19 wherein forming comprises defining a maskwith an opening therein at the location of the trench and plasma etchingthough the mask opening to form the narrow elongated trench.
 24. Themethod of claim 19 wherein the narrow elongated trench has a width atthe surface of the substrate of about 1 micrometer.
 25. The method ofclaim 24 wherein the narrow elongated trench has a depth from thesurface of the substrate of about 9 micrometers.
 26. The method of claim19 wherein closing comprises depositing a layer of material to close thetop port substrate surface opening.
 27. The method of claim 26 whereinlayer of material is a material taken from the group consisting of apolysilicon, a nitride or an oxide.
 28. A method for formingmicrochannels, comprising: forming a monocrystalline silicon layer overa monocrytalline silicon substrate; forming a narrow elongated trenchthrough the monocrystalline layer and into the monocrystalline siliconsubstrate; performing an etching of a base region of the narrowelongated trench to form a microchannel structure having a top portopening; and closing the top port opening of the microchannel structureto entirely enclose the microchannel structure.
 29. The method of claim28 wherein closing comprises growing monocrystalline silicon to closethe top port opening in trench above the formed microchannel structureand produce the microchannel structure enclosed completely inmonocrystalline silicon.
 30. The method of claim 28 wherein performingcomprises anisotropically wet etching the base region to define themicrochannel structure with a generally rhombohedral cross-sectionalshape.
 31. The method of claim 30 wherein the anisotropic wet etch ismade using a TMAH solution.
 32. The method of claim 30 wherein theanisotropic wet etch is made using a KHOH solution.
 33. The method ofclaim 28 wherein forming the narrow elongated trench comprises defininga mask with an opening therein at the location of the trench and plasmaetching though the mask opening to form the narrow elongated trench. 34.The method of claim 28 wherein closing comprises depositing a layer ofmaterial to close the top port opening.
 35. The method of claim 34wherein layer of material is a material taken from the group consistingof a polysilicon, a nitride or an oxide.